An update to the Cassini Tour Page

Where's the Cassini Saturn orbiter going to be in the next week -- or hundred weeks? It's all already planned out. Cassini will remain in Saturn orbit for the rest of its mission; but did you know that mission planners have already determined the exact, looping path Cassini will take between now and its plunge into Saturn's atmosphere on September 15, 2017? And of course they determined this path knowing already where all of Saturn's moons would be at every moment during the same period.

Since the mission started I've maintained a page on "Cassini's Tour of the Saturn System" that lays out the plans for Cassini's mission in excruciating detail. I recently received updated information on Cassini's trajectory so I rebuilt the page; lots of flyby times and distances changed from my previous version of the page, but not by very much.

The page is dense with detail, so I thought I'd take a little bit of it and explain what some of the details mean, and what the capabilities and limitations of the tour page are.

There are two bits of business I have to get through before I begin. The first is some basics on the shape of an orbit. Each orbit is elliptical. Elliptical orbits have an apoapsis -- the point at which Cassini is farthest from Saturn -- and a periapsis -- the point at which Cassini is closest to Saturn. The Cassini team increments the orbit count each time Cassini reaches its apoapsis, which works well because that is when the spacecraft is moving the slowest and is least likely to be in the middle of any major event. (There's only one major mission event that unfolded while Cassini crossed through apoapsis: the December 31, 2004 flyby of Iapetus, the most distant of Saturn's major round moons.) Near periapsis, Cassini is often very busy, taking high-resolution observations of rings, moons, atmosphere, magnetic field, and everything else. Here, taken at random from Cassini's history, is what one orbit looked like, Rev 19, which took place from December 11, 2005 to January 5, 2006.

NASA / JPL / Solar System Simulator (David Seal)

Cassini's Rev 19

The other piece of business is that I have to explain something convoluted about how Cassini's orbits are named. The orbits are called "Revs" by the team. There was a lot of science performed during what the mission called "Rev 0." Rev 0 included all the science from Cassini's initial approach to Saturn, death-defying orbital insertion maneuver, and the subsequent two months of its first orbital leg out from Saturn. You might think that once Cassini reached apoapsis on this very first orbit, the next orbit would be called "Rev 1." That was the original plan. But due to a hardware problem discovered with the Huygens probe, they had to replan the way the probe mission was conducted. This could potentially have caused the replanning of the entire Cassini mission, which would have been disastrous (and possibly even impossible). But the clever, clever navigators figured out a way to have Cassini enter orbit as planned on July 1, 2004 and then complete three orbits in place of the originally planned first two orbits, in such a way that Cassini wound up with nearly the same position and velocity by the end of the third orbit that they would have at the end of the originally planned second orbit.

This solution permitted them not to throw out all the planning they had already done for Revs 3 onward, but presented them with a naming problem. If they renamed the Revs, it would throw their planning into confusion -- plans that had already been made for something called "Rev 4" would later be called "Rev 5", and there would be major potential for errors as people got confused in old documentation and wondered which Rev they were really talking about. So, instead of renumbering the whole mission, the plans for Revs 1 and 2 were replaced with three new orbits called Rev A, Rev B, and Rev C. So the orbits are numbered as follows: Rev 0, Rev A, Rev B, Rev C, Rev 3, Rev 4, Rev 5...and on up to Rev 293. Therefore Rev 19, which I describe below, was actually Cassini's 20th orbit of Saturn.

All right. Now that that's out of the way, here is the piece of my tour table that describes Rev 19.

You can see that it begins on December 11, 2005, with "Apoapsis Rev 19" -- that's where Rev 19 starts, with Cassini at its farthest point from Saturn on that orbit. The date is reported again in a different way, "2005-345T04:27" -- "345" this is the Julian date, which counts up the days from January 1. Dates and times are reported in Universal Time, according to the clock on the spacecraft. If you use the Solar System Simulator to see where Cassini is at any given moment, or if you use any number of tools to search for Cassini images, they also use times in UTC as recorded by the spacecraft clock.

There's more information provided about Cassini's Rev 19. It says: "period = 27.4 days, inclination = 0.4°, r = 53.563 Saturn radii, phase = 89°" All of these parameters describe Cassini's orbit at that instant. If no other forces acted on the spacecraft, it would take 27.4 more days to complete the orbit and return to its apoapsis. The orbit is inclined to the plane of Saturn's rings by a very tiny 0.4 degrees, so Cassini views the rings nearly edge-on. At the moment of apoapsis, Cassini was at a position that was 53.563 Saturn radii from Saturn's center -- Saturn radii (1 Saturn radius = 60,330 kilometers, the elevation at which Saturn's atmospheric pressure is 1 bar) are a convenient distance unit often used by the team within the Saturn system, and make it easier to visualize Cassini's position than the number reported in kilometers. The 89-degree phase explains how much of Saturn appears to be lit by the Sun when Cassini is at apoapsis -- in this case, Saturn is almost exactly half-full. So Cassini saw a half-full Saturn with the rings appearing as a skinny line as it fell toward the planet on the inbound leg of Rev 19. And, here you go, here's what the view looked like.

The next item in the table isn't for nearly two weeks. Things on highly elliptical orbits move very slowly near apoapsis, very quickly near periapsis. Cassini was probably quite busy during that time taking data, there just wasn't a lot happening in terms of its orbital mechanics. The next thing you see is the "ascending ring plane crossing." Because the plane of the rings contains the center of Saturn's gravitational field, and the plane of Cassini's orbit also contains the center of Saturn's gravitational field, and the two can't possibly be perfectly coincident, there are two points on each and every orbit where Cassini passes through Saturn's ring plane. One crossing happens between apoapsis and periapsis, and the other one happens between periapsis and the next apoapsis. One of these is the ascending ring plane crossing, where Cassini travels from the south side of the rings to the north side of the rings, and the other is the descending ring plane crossing, where Cassini travels from north to south. In this case, Cassini crossed the ring plane from south to north. On an orbit like this one, which is only barely inclined to the ring plane, the crossing doesn't make a whole lot of difference in terms of the appearance of the images.

(I'm repeating the table here so you don't have to keep scrolling up and down.)

Next up is a cluster of events within a few hours of periapsis. Periapsis happened at 21:23 on December 24, 2005. At the time, Cassini was at a distance of 4.572 Saturn radii from Saturn's center, which is just a little bit inside the orbit of Tethys. Thus it approached Saturn closer than Tethys, Dione, Rhea, or Titan ever do, but didn't get as close as Enceladus or Mimas. Now, just because it was near the orbit of Tethys doesn't mean that Tethys happened to be near Cassini in its orbit at the time, and in fact, it wasn't. The moons all follow their own paths on their own times, and Cassini happened to pass within 120,000 kilometers (the arbitrary cutoff used to generate this table, about 2 Saturn radii) to just four of them: Enceladus (94053.5 km), Pallene (107286.4), Telesto (19275.4 km), and Helene (74718.3 km). (Yes, the amount of precision reported on these numbers is a little extreme. It's just what the software spits out.)

These reasonably close approaches are called "nontargeted flybys." What that means is that the mission planners didn't intentionally line up Cassini to fly past these moons at this time; these moons just happened to be in the neighborhood when Cassini passed by. Sometimes, the science team chooses to take advantage of these chance encounters, but sometimes there are more compelling things to do. In this particular case, I checked the archive of Cassini's images for Rev 19 by searching the data at the PDS Rings Node, and the mission team took the opportunities to image Enceladus and Telesto, but not Pallene or Helene. I should note that it's no surprise they didn't image Pallene, because not only was it a very distant pass by a very tiny moon, but Pallene was only discovered in June 2005, and there might not have been time by December to include images of it in Cassini's science plans even if it was a good viewing opportunity.

Of course, lots of Cassini's moons look very nice even if Cassini is farther than 120,000 kilometers away from them, and in fact Cassini was very busy with imaging during the period around periapsis on Rev 19, taking very nice photos particularly of Hyperion, Rhea, Dione, Tethys, and this very cool Epimetheus and Janus mutual event, among other things. So the tour table doesn't tell you all the details of what Cassini was observing.

About two days after periapsis, Cassini finally gets out as far as Titan's orbit, and meets it in a "targeted" flyby. Targeted flybys are ones that the mission actually spends maneuvering fuel in order to get to. Targeted flybys of Titan are very, very important, and not only because Titan is scientifically interesting. It's also because Titan is the only moon in the Saturn system big enough to bend Cassini's trajectory with its gravity, and Cassini's clever navigators use such Titan gravity assists to steer Cassini around, changing the shape of its orbit. This particular targeted flyby, at an elevation of 10410.9 km above the center of Titan, is more distant than usual; most are at an elevation of about 1000 kilometers. There are two codes after the text "targeted flyby of Titan:" "(19TI [T9])". These represent two different ways of numbering satellite encounters. "19TI" is simply the Rev number followed by a two-letter abbreviation for the moon. But targeted flybys get a second codename, in this case "T9." This was the ninth originally planned targeted Titan flyby, though since there were targeted Titan flybys during Revs A, B, and C, it was, of course, the tenth actual flyby of Titan. Just to make things more confusing. I wouldn't bother with these T numbers but they are often used in scientific papers describing data from Cassini's targeted Titan encounters in lieu of calendar dates, particularly by the RADAR team, so it's useful to have a reference to which dates correspond to which code names.

After the targeted encounter with Titan, things slow down. The next event is Cassini's crossing back through the ring plane, from north to south this time, and it finally reached apoapsis again, starting the next Rev, Rev 20, at 14:09 on January 5, 2006. Now, if you compare this entry for apoapsis to the previous one, you'll see that they're different. I'll write the two again, right next to each other:

Their inclinations are the same, but other than that they are different: different distance from Saturn, different phase, different period. You can see this in the orbital diagram I posted at the very top of this entry, that Cassini's position at apoapsis on Rev 20 didn't quite get out as far from Saturn as its apoapsis on Rev 19. Some of this change in orbit is accomplished with thruster firings (which I don't include in the table), but most of the change in the shape of the orbit was done using the gravity-assist flyby of Titan. In this case, they used Titan to shrink the orbit, which also made it faster, plus they pulled the orbit five degrees to the west, making Cassini see a less-fully-lit Saturn than it saw at the previous apoapsis.

So now hopefully I've explained most of what you can see in the full "Cassini's Tour of the Saturn System" table. Scroll up and down through it and see if you can see how nontargeted encounters are clustered around periapsis. See how there's one ring plane crossing for each segment between a periapsis and an apoapsis. See if you can find the couple of times that Cassini circularizes its orbit so that it meets Titan once on each side of Saturn, 8 days apart, and you miss a periapsis or an apoapsis. See if you can find periods of time where Cassini does not encounter Titan for a long period, so its orbit remains the same shape for several Revs in a row. Watch how the inclination of the orbit gets pumped upward and then downward over time. Watch how the orbit tends to grow smoothly and shrink smoothly with time, and how it wanders upward and downward in phase.